In order to study the origin of late potentials, their distribution was analyzed in 16 patients who had undergone surgery for arrhythmia due to ventricular tachycardia following myocardial infarction. The potentials were measured in sinus rhythm using 63 unipolar leads placed on the chest before the operation, then on the epicardium and endocardium during the operation. Epicardial and/or endocardial activity extending beyond the QRS complex measured from unfiltered chest signals and characterized by slowed propagation at the edge or inside of necrotic regions, stable from one beat to the next, and showing simple (39 per cent), double (34 per cent) or fragmented (27 per cent) deflections on the electrocardiograms were observed in 5/6 patients without bundle-branch block and in 5/10 patients with block. Similar activity but which did not extend after the QRS was detected in the 6 other patients. For signals filtered at 55 Hz, a close correlation between the distribution of chest, epicardial and endocardial potentials was observed, thus allowing approximate location of the origin of late potentials from the chest. Anterior or apical sites corresponded to close extrema in the precordial region, whereas the other sites were associated with more distant extrema. An analysis of potential distribution thus gives a better understanding of the electrogenesis of late potentials as well as their detection on the chest.